Dept. of Chemical and Biomolecular Engineering
http://hdl.handle.net/2142/9130
Sun, 24 May 2015 13:11:39 GMT2015-05-24T13:11:39ZInfrared Photon Initiated Conformer Interconversion, Fourier Transform Microwave Spectroscopy of Hydrogen Bonded Trimers, and Study of Conformer Cooling in Supersonic Expansions
http://hdl.handle.net/2142/77318
Infrared Photon Initiated Conformer Interconversion, Fourier Transform Microwave Spectroscopy of Hydrogen Bonded Trimers, and Study of Conformer Cooling in Supersonic Expansions
C-H stretch fundamentals were excited by an IR OPO laser in 2 molecules having conformers. Because the barrier for interconversion is lower (higher) in methyl vinyl ether (methyl nitrite) than IR photon energies, IR fluorescence spectra showed conformational interconversion occurred (did not occur). For the first time, well defined energy in collisionless conditions was deposited in a ground electronic state molecule with subsequent intramolecular energy flow perturbing the equilibria between conformers (methyl vinyl ether).; A FT-MW spectrometer is used for assignment of spectra of H-bonded trimers HCN-HCN-HCN, (HCN)$\sb2$Y (Y = HF, HCl, HCF$\sb3$, CO$\sb2$) and X-(HCN)$\sb2$ (X = NH$\sb3$, H$\sb2$O, CO, N$\sb2$). Ground vibronic state geometries and shrinkages of the H-bonds (compared to dimers) are obtained. Also obtained is the spectrum of Ar(HCN)$\sb2$, a T-shaped trimer with Ar about equidistant from the c.m. of each HCN, and the (HCN)$\sb2$ subunit essentially equivalent to free (HCN)$\sb2$.; Theoretical treatment of measured induced dipole moments in (HCN)$\sb2$ and (HCN)$\sb3$ show they are well predicted by the first 4 nonzero molecular electrical multipole moments and bond polarizabilities of HCN. Equations are derived for the induced moment per HCN in an infinite chain of HCN's. These allow separation of contributions to the induced dipole moment per HCN from same-chain and off-chain sites in HCN crystal.; $\sp&#x192;$Kr quadrupole coupling constants for $\sp&#x192;$Kr-pyridine and $\sp&#x192;$Kr-CO$\sb2$ are calculated from proposed representations of charge on pyridine and CO$\sb2$. Experimental coupling constants therefore test descriptions of charge distributions of pyridine and CO$\sb2$.; Dependence of conformational relaxation in ethanol, isopropanol, and ethyl formate in supersonic expansions on noble carrier gas type show He will not relax the high energy conformers, Ar and Kr will relax them completely, and Ne will relax the alcohols but not ethyl formate. Results for the high energy isomer of CO$\sb2$-HCN and of DF-HF(HF-DF) show the number ratio of high:low energy isomer is perturbed in different carrier gases, with He yielding a "freeze-in temperature" equal to the nozzle temperature, but Ar causing more relaxation. Mechanisms for such relaxation are suggested.
Chemistry, Physical
Fri, 01 Jan 1988 00:00:00 GMThttp://hdl.handle.net/2142/773181988-01-01T00:00:00ZClose-Coupling and Coupled States Scattering Calculations Applied to Collisions of Methane With Rare Gases
http://hdl.handle.net/2142/77316
Close-Coupling and Coupled States Scattering Calculations Applied to Collisions of Methane With Rare Gases
Close-coupling and coupled states scattering calculations are applied to helium methane and neon-methane collisions. State-to-state and total differential cross sections calculated in the coupled states approximation are found to be in at least rough qualitative agreement with the close-coupling results, making the coupled states approximation a useful tool for analyzing particular scattering systems. The theoretical cross sections are compared to experimental measurements and are found to agree in part. The close-coupling calculations of helium-methane scattering show that some disagreements which are observed between the measured helium-methane differential cross sections and coupled states calculations cannot be due to the coupled states approximation. The close coupling calculation of neon-methane scattering and the failure of a thorough search for a neon-methane interaction potential which will fit theory fully to experiment show that disagreements which are observed between the measured neon-methane differential cross sections and coupled states calculations can be due neither to the coupled states approximation nor to the interaction potential. Comparison the results of coupled-states calculations which include closed rotational channels to the results of calculations which neglect them show that these closed channels have a slight influence on the calculated cross sections. The neglect of vibrational closed channels is suggested to be a possible source of disagreement between theory and experiment.
Chemistry, Physical; Physics, Molecular
Fri, 01 Jan 1988 00:00:00 GMThttp://hdl.handle.net/2142/773161988-01-01T00:00:00ZComputational Studies in Chemical Quantum Physics
http://hdl.handle.net/2142/77315
Computational Studies in Chemical Quantum Physics
Two stochastic dynamic models are used to study several aspects of curve crossing phenomena in dissipative systems. A surface hopping model is used to test the qualitative predictions of earlier theories. The simulation results agree well with the qualitative picture. Results are obtained for an alternate semiclassical model based on a vector spin representation which is derived via a variational principle. The vector model shows some differences in behavior as compared to the hopping model. In certain regimes the vector model shows chaotic behavior.; A numerical path integral technique based upon a quasiclassical Langevin equation is presented for the calculation of quantum mechanical properties of a system coupled to a dissipative bath. Fully quantum mechanical results are obtained by decorating quasiclassical paths with quantum fluctuations. The use of Diophantine integration has been compared to Monte Carlo sampling of the fluctuation variables. Results are presented for several model systems.
Chemistry, Physical
Thu, 01 Jan 1987 00:00:00 GMThttp://hdl.handle.net/2142/773151987-01-01T00:00:00ZStudies on Aperiodic and Quasiperiodic Crystals
http://hdl.handle.net/2142/77317
Studies on Aperiodic and Quasiperiodic Crystals
In this work Fixman's self-consistent phonon theory has been extended and applied to one- and two-component aperiodic and quasiperiodic crystals. First the free energy as a function of density is calculated using an average environment approximation. Then this approximation is lifted and all particles are allowed to move and search out their local free energy minima. Upon relaxation it was found that the one-component quasicrystal undergoes a structural transition to an aperiodic crystal near $\rho \cong$ 1.03. In the binary quasicrystal there was not a pronounced structural transition, but rather local regions of stability and instability. In both the one- and two-component aperiodic crystal it was found that the quenched free energy was dependent upon the density as well as the number of particles in the system. Lastly, the quasicrystalline lattice was transformed into a lattice of water molecules. Molecular dynamics was used to determine that the lattice was unstable.
Chemistry, Physical
Fri, 01 Jan 1988 00:00:00 GMThttp://hdl.handle.net/2142/773171988-01-01T00:00:00ZPhotodissociation Dynamics of Small Molecules
http://hdl.handle.net/2142/77313
Photodissociation Dynamics of Small Molecules
Energy disposal into CN (product) degrees-of-freedom following photolysis of BrCN and NC=CN (cyanogen) upon absorption of a single 193 nm photon was investigated. Supersonic expansion in a molecular beam was utilized to obtain a narrow distribution of initial parent states. Laser induced fluorescence under collisionless conditions was used in determining the product internal state distributions. It was found that for both BrCN and cyanogen the disposal of energy into the product degrees-of-freedom is strongly influenced by the initial parent rovibrational excitation. Relative rotational state population of 0.15 for the v = 1 state of CN and no significant population of the v = 2 state in photolysis of cyanogen was observed. The population of rotational states of CN in the v = 0 and v = 1 vibrational states follows the prediction of a phase space theory model, while the vibrational state populations are nonstatistical and believed to be governed by the Franck-Condon factors. Angular and velocity distributions of state resolved (single rotational state) fragments were obtained with a uniquely designed detector array in conjunction with laser induced fluorescence. The obtained angular distributions are isotropic for rotational states in both v = 0 and v = 1 vibrational states of the CN fragments indicating long lifetime of the parent cyanogen state compared to the molecular rotation period. With the increase of internal energy (higher rotational state) of the fragment which is being detected the corresponding velocity distributions become narrower and the maximum in the distributions shifts towards lower velocities. The experimentally obtained velocity distributions were fitted to a phase space theory model assuming that the excess energy and the total angular momentum are conserved. It was necessary to allow for symmetric as well as non-symmetric disposal of the angular momentum into the two CN fragments in order for the experimental velocity distributions to fit the model.
Chemistry, Physical
Tue, 01 Jan 1985 00:00:00 GMThttp://hdl.handle.net/2142/773131985-01-01T00:00:00ZSelf-Consistent Phonon Theory of Aperiodic Solids and Density Functional Theories of Freezing and Vitrification
http://hdl.handle.net/2142/77314
Self-Consistent Phonon Theory of Aperiodic Solids and Density Functional Theories of Freezing and Vitrification
We extend Fixman's self-consistent phonon theory of the thermodynamic properties and fluctuations of the hard sphere crystal to an amorphous lattice. In doing so, we develop a theory of lattice vibrations in amorphous materials via a perturbation theory around the Einstein oscillator approximation. This development is analogous to the mean spherical model description of electronic excitations in liquids. The theory exhibits a mechanical instability rather close to computer simulated glass transitions, suggesting the possibility that the simulations probe only linear stability because of the short simulation times.; In addition, a density functional theory of freezing into an aperiodic lattice is presented. With use of free energy functionals for inhomogeneous hard-sphere fluids, the stability of a density wave with the structure of Bernal random packing is evaluated. Although the properties of the transition are sensitive to the form of the direct correlation function, the present calculations indicate a limit of metastability at a density (rho) = 1.03. The frozen lattice becomes more stable than the liquid at a density (rho) = 1.14.; Finally, with analogy to the "highly accurate" summation of cluster diagrams for hard sphere fluids a la Carnahan-Starling, we present for arbitrary potential systems, based on the generalization of the second virial coefficient to inhomogeneous systems, which, when applied to hard-sphere, soft-sphere, and Lennard-Jones freezing, yield melting characteristics in remarkable agreement with experiment. Implications for the liquid-glass transition in all three potential systems are also presented.
Chemistry, Physical
Tue, 01 Jan 1985 00:00:00 GMThttp://hdl.handle.net/2142/773141985-01-01T00:00:00ZTunneling Systems in Condensed Phases: Quantum Dynamical Monte Carlo and Analytical Theories
http://hdl.handle.net/2142/77310
Tunneling Systems in Condensed Phases: Quantum Dynamical Monte Carlo and Analytical Theories
A Monte Carlo method for the evaluation of real time path integrals at finite temperatures is proposed. The technique is applied to two level tunneling systems coupled to many body environments, models for electron transfer and for the quantum coherence phenomenon in Josephson junctions. A dynamical correlation function for the latter shows a plateau indicative of the localization effects of damping.; An optimized random phase approximation is derived from an expansion of the free energy in terms of the generating functional. The theory is applied to a two level system coupled to a Gaussian bath. Short time behavior and long time averages of correlation functions are reproduced well.; A general method of renormalization of influence functional bonds for the Monte Carlo calculations is examined. It was hoped that the scheme would reduce significantly the problem of phase cancellation that is the major limitation on the real time Monte Carlo method. Unfortunately the algorithms did not seem to improve convergence to any great extent.; Using a complex time correlation function that greatly improves the convergence properties of the Monte Carlo method, results for the two level system coupled to a dissipative bath are presented. Defects and advantages of various analytical theories are highlighted by comparison to the (statistically) exact results of the Monte Carlo.; Equations are derived for influence functional bonds due to a damped harmonic bath whose initial and/or final points are fixed, a possible importance sampling route for the calculation of rate constants.
Chemistry, Physical
Tue, 01 Jan 1985 00:00:00 GMThttp://hdl.handle.net/2142/773101985-01-01T00:00:00ZVibrational Dynamics in Disordered Molecular Crystals by Picosecond Coherent Raman and Photon Echo Spectroscopies (Exciton, Low Temperature)
http://hdl.handle.net/2142/77312
Vibrational Dynamics in Disordered Molecular Crystals by Picosecond Coherent Raman and Photon Echo Spectroscopies (Exciton, Low Temperature)
An understanding of vibrational dynamics in complex molecules is a fundamental problem in chemical physics and is important for discerning how vibrational relaxation effects actual chemical reaction rates. The natural states of a pure crystal, however, are delocalized excitons and are described by the language of solid state physics. In this thesis the important processes involved in vibrational exciton (vibron) dynamics are differentiated by investigating vibron relaxation in isotopically and chemically disordered crystals at low temperature by picosecond time-delayed Coherent Anti-Stokes Raman Scattering (psCARS) and photon echo spectroscopy (PE).; Picosecond laser spectroscopy is used to observe vibrational dynamics directly in the time domain. PsCARS is very useful for the investigation of pure and heavily disordered crystals since the Raman effect has a small cross section and consequently low net absorption. The photon echo measurements are used to examine vibron relaxation above the electronic excited state and can be used on very dilute chemically mixed crystals since the laser induced transitions are dipole allowed. The experiments performed were all done at low temperature (1.5(DEGREES)K and 10(DEGREES)K) where the vibrational relaxation rates are slowed allowing the effect of crystal disorder to be investigated.; The results of this study have shown that vibrational relaxation in low temperature molecular crystals depends on a variety of effects. In a pure crystal the effect of molecular symmetry has been shown to be important in determining whether fundamental or combination vibrations are involved in vibrational relaxation. The studies of isotopically mixed crystals have shown that one and two site processes can be of importance and have shown that relaxation is greatly affected by a lower energy state of the same normal coordinate on the heavier isotope. In addition, the photon echo experiments look at isolated guest molecules in a chemically different host and have shown that relaxation of the guest can be slower than the host modes of nearly equal energy, thus indicating single site decay with the emission of bulk phonons.
Chemistry, Physical
Tue, 01 Jan 1985 00:00:00 GMThttp://hdl.handle.net/2142/773121985-01-01T00:00:00ZRism Methodology and Its Application to Orientational Correlations and to Excess Electrons in Liquids
http://hdl.handle.net/2142/77311
Rism Methodology and Its Application to Orientational Correlations and to Excess Electrons in Liquids
A theory is presented for the orientational pair correlation factor, g(,2), of a molecular liquid in the interaction site picture. It is based on the so-called proper integral equation theory of molecular fluids and leads to a cluster series expansion for g(,2). A topological reduction which removes the bare Mayer f-bonds in favor of the appropriate pair correlation bonds is performed on this exact series.; The theory is applied to a hard sphere model of CS(,2). The lowest-order terms in a molecular ordering scheme are evaluated exactly by numerical integration. The results agree well with experiment and computer simulation for 192K < T < 298K.; Next a simple model for an electron in a nonpolar solvent is discussed. This problem can be shown to be isomorphic with that of a polymer molecule in solution in the discretized path integral representation. A self-consistent theory for the electron's chemical potential is described in terms of solvent-induced interactions between sites on the isomorphic polymer. Calculation of the equilibrium structural and thermodynamic properties of the electron requires evaluation of non-Gaussian path integrals. These are performed using Feynman's polaron approximation.; For a model hard sphere system characterized by the hard sphere diameter, (sigma), and at a temperature for which the electron's thermal wavelength (lamda)(,e) = 6, a localization transition is found at the reduced density (rho)(sigma)('3) (DBLTURN) .3. The transition is sharper at lower temperatures. Calculations of the electron's correlation length, chemical potential, imaginary time response function, and of the electron-solvent pair correlation function are presented for a variety of temperatures and solvent densities.
Chemistry, Physical
Tue, 01 Jan 1985 00:00:00 GMThttp://hdl.handle.net/2142/773111985-01-01T00:00:00ZDiffusion and Evaporation of Nitrogen on Tungsten (100)
http://hdl.handle.net/2142/77309
Diffusion and Evaporation of Nitrogen on Tungsten (100)
Surface diffusion and evaporation from a layer of nitrogen chemisorbed on the (100) plane of tungsten has been examined using Auger spectroscopy. The initial localized deposit required for diffusion measurements in a concentration gradient is created by a highly collimated molecular beam. The sample, under computer control, is moved across an electron beam, and the Auger spectra are recorded automatically at different points on the surface. Nitrogen diffusion over the (100) tungsten surface was found to be concentration independent, taking place over a barrier of 28 kcal/mole with a prefactor D(,0) of 3.0 x 10('-2)cm('-2)/sec. At the temperatures 900 < T < 1050 K necessary to observe spreading on the order of 50-100 (mu)m in the concentration profile evaporation was found to be a competing process. Therefore, the desorption kinetics of (beta) chemisorbed nitrogen on W(100) was studied by a combination of Auger spectroscopy and flash desorption. The activation energy for desorption was found to be 82 kcal/mole with a prefactor of 0.01 molecules/(cm('2)sec). A comparison with the diffusion of nitrogen on the W(110) surface is made, showing for the first time the effect of surface structure on the diffusion of a chemisorbed gas on a solid surface.
Chemistry, Physical
Sun, 01 Jan 1984 00:00:00 GMThttp://hdl.handle.net/2142/773091984-01-01T00:00:00Z